In the past, in optical communication, optical fiber that can transmit high capacity data at high speed over long distances has been used as a transmission medium. Photoelectric conversion modules of various types have been developed and practically implemented that convert an optical signal arriving by transmission from such an optical fiber into an electrical signal using photoelectric conversion element such as a photo-diode or the like, and that then output that electrical signal to the exterior.
Now, a typical such photoelectric conversion module will be explained. As shown in FIGS. 25 and 26, this photoelectric conversion module 101 comprises an optical lens (optical window) 106 that regulates the optical signal arriving by transmission from an optical fiber 105, a photoelectric conversion element 108 that converts the optical signal into an electrical signal, and a signal output section 121 that outputs this electrical signal to the exterior, etc. An output electrode 108a of the photoelectric conversion element 108 and a center conductor 121a of the signal output section 121 are electrically connected by a metallic wire 115, and thereby the electrical signal is outputted to the exterior of the module 101.
And, with the photoelectric conversion module 101A shown in FIGS. 27 and 28, an amplifier 130 is provided between the photoelectric conversion element 108 and signal output sections 121. This amplifier 130 is an amplifier that amplifies the single phase electrical signal that has been produced by photoelectric conversion. The output electrode 108a of the photoelectric conversion element 108 and an input electrode 130a of the amplifier 130 are electrically connected with a metallic wire 132, and output electrodes 130b of the amplifier 130 are electrically connected to the center conductors 121a of the signal output sections 121 by metallic wires 133.
With the photoelectric conversion module 101 described above and shown in FIGS. 25 and 26, the output electrode 108a of the photoelectric conversion element 108 and the center conductor 121a of the signal output section 121 are directly connected by the metallic wire 115. And, with the photoelectric conversion module 101A shown in FIGS. 27 and 28, the output electrode 108a of the photoelectric conversion element 108 and the input electrode 130a of the amplifier 130 are directly connected by the metallic wire 132, while the output electrodes 130b of the amplifier 130 and the center conductors 121a of the signal output sections 121 are directly connected by the metallic wires 133. Each of the metallic wires 132 and 133 has an inductance component, and this is a high impedance as compared with the characteristic impedances of the output electrode 108a of the photoelectric conversion element 108, of the output electrodes 130b of the amplifier 130, and of the signal output sections 121. Due to this, there has been the problem that reflection of the electrical signal takes place because of this impedance mis-matching, in particular in the high frequency band, so that the high frequency characteristic deteriorates.
Thus, with the photoelectric conversion module 102 described in Patent Document #1, as shown in FIG. 29, there is included a transmission line substrate 140 that outputs the electrical signals after conversion to the exterior of the module 102, and a substrate 120 upon which an impedance matching circuit 110 is installed is provided between the amplifier 130 and the transmission line substrate 140. This impedance matching circuit 110 comprises parallel flat plate micro chip capacitors 112 that have capacitance components, and metallic wires 114 that are electrically connected between the amplifier 130 and the capacitors 112 and between the capacitors 112 and the transmission line substrate 140, each of these metallic wires 114 having an inductance component.
An equivalent circuit for the impedance matching circuit 110 described above is shown in FIG. 30. The capacitance component 112a is adjusted by varying the dimensions of the capacitor 112 and the material of which it is made, while the inductance components 114 are adjusted by varying the lengths of the metallic wires 114. It is possible to anticipate impedance matching with respect to the transmission line substrate 140 in this manner, and so it is possible to improve the high frequency return loss characteristic of the photoelectric conversion module 102, and it is possible to improve the transmission characteristics for electrical signals.
Patent Document #1: Japanese Laid-Open Patent Publication 2002-353493.
However, since this equivalent circuit for the impedance matching circuit 110 of the photoelectric conversion module 102 of Patent Document #1 is similar to the circuit for a single stage low pass filter as shown in FIG. 30, there is an anxiety that the transmission loss in the high frequency band will increase, and that the flatness of the transmission characteristic will be damaged. Thus, while there is a need to set the capacitance component 112a and the inductance component 114a to values one level higher in order to improve the transmission characteristic, if this is done, there is an anxiety that the phase linearity will not be maintained, and that this will invite deterioration of the group delay.
Therefore, an object of the present invention is to provide a photoelectric conversion module which can reduce return losses due to impedance mismatching and also enhance the flatness of the transmission characteristic in the high frequency band, and which can stabilize the group delay characteristic by maintaining the phase linearity.